1. Field of the Invention
This invention relates to an output circuit of a semiconductor integrated circuit, and more particularly to the improvement for reduction in the power source noise occurring when a plurality of output circuits perform switching operations at the same time.
2. Description of the Related Art
It is well known in the art that resistive, capacitive and inductive loads are parasitically associated with the power source line of a semiconductor integrated circuit (IC). Among these parasitic loads, the inductive load in particular causes a large noise level when current flowing via the power source line changes abruptly. Such an abrupt change of current is caused by an output circuit for outputting a signal to the exterior of the IC. That is, it is necessary for the output circuit to extract a sufficiently large current from the power source line and charge or discharge a load capacitor outside the IC so as to drive the load capacitor at high speed. In general, a plurality of output circuits of the same type as described above are used in the IC, and some of them perform switching operations at the same time.
FIG. 6 is a circuit diagram showing the construction of the prior art output circuit provided in the MOS IC. Input signal In in the IC is supplied to buffer 24 having a CMOS inverter constituted by P-channel MOS transistor 22 and N-channel MOS transistor 23 via pre-buffer 21 constituted by an inverter. Output signal Out is output from the output terminal of buffer 24.
In the above output circuit, buffer 24 is formed of one P-channel MOS transistor and one N-channel MOS transistor. Therefore, the current flowing out from power source voltage terminal Vcc and current flowing into ground voltage terminal Vss when output signal Out is changed are determined by the characteristics of the above transistors.
Recently, with increases in the output currents and operation speeds of ICs, it has become necessary to increase the mutual conductance of the output circuit in order to enhance the current driving ability for the load. To meet this requirement, the ON-resistances of transistors 22 and 23 constituting buffer 24 tend to be reduced.
As a result, the possibility of noise occurring in the power source lines for the power source voltage and ground voltage becomes larger, and the possibility of noise being mixed into the signal of the output circuit which does not perform the switching operation also increases.
Conventionally, the buffer of the output circuit is constituted by transistors having a large current driving ability, and the transistor is operated at a high switching speed. As a result, the noise occurring in the power source line increases even further, thereby causing the introduction of more noise into the signal of the output circuit.